If I were in a 2-on-2 battle, and used a powerful move against one opponent, and they used mirror coat/counter and sent it to my other pokemon, who also used this move aginst their 2nd pokemon, who uses it against my pokemon. Would this mean that the original damage is 8 times the original, even if this is very unlikely?

In simple terms, can use use mirror coat against mirror coat? or counter a counter attack?

Now it's the end of the turn. If Pokemon B chooses counter now, it will go last, therefore it will now counter the next move that Pokemon A does. If both choose Counter in the same move, nothing happens.